It is common in CATV distribution systems to use a broadband signal (e.g. 50 to 1000 MHz) to carry the various channels and other information to the subscribers, another broadband signal (e.g. 5 to 40 MHz) to carry information from the subscribers to the cable distribution station, and 60 Hz single phase power to operate amplifiers and other devices located at various points on the cable system. In such systems, the broadband signals and AC power are typically each carried on the same transmission line, e.g., the center conductor and braid of a coaxial cable.
Each of the broadband signals originate from a central location. The coaxial cables used to carry these signals inherently have loss characteristics. Thus, amplifier stations must be installed at appropriate locations along the cable in order to compensate for the losses and deliver faithful broadband ("RF") signals. The single phase AC power signal is needed to operate the amplifier stations.
The power signal is passed along the cable concurrently with the RF signal. The power level of the AC signal is much greater than that of the RF signal, and uses different and separate circuitry to operate the amplifier station. Therefore, the AC power signal must be separated from the RF signal at each of the amplifier stations.
It is common practice to use an RF choke and a capacitor to separate the single-phase AC power signal from the broadband RF signals at points along the cable where the RF signal is to be processed in an RF device. After passing the device, the AC power is recombined with the broadband signal, requiring the use of a second RF choke.
The AC power has a current magnitude up to 15 amperes at 60 volts. On the other hand, the broadband RF signal has a low peak voltage of about 0.3 volts. When isolating the AC power from an RF device, the chokes must prevent the RF broadband signal from passing through the choke along the AC power path in order to avoid a significant loss of signal.
One choke, developed by the present inventor which solves many of the problems in the art, is described in U.S. Pat. No. 5,032,808 which is incorporated herein by reference. The choke described in the '808 patent comprises a series of three sets of windings, each of which has a distinct number turns wound upon a core having a uniform cross sectional area along its entire length. The first set of windings is connected at one end to an input lead and at an opposite end to the second set of windings through an air coil inductor. The second set of windings is similarly connected to one end of a third set of windings by means of a second air coil inductor. The output end of the third set of windings in turn is connected to an output lead.
A commercial RF choke is typically constituted by a number of turns of insulated copper wire wound upon a ferrite coil form. A resistor can be connected in parallel with a portion of this wire coil, e.g., from a preselected turn to one of the lead wires, to serve as a shunt. This parallel resistor is selected so that it does not significantly reduce the impedance of the RF choke. There is an effective capacitance between turns of the wire coil, which produces a self-capacitance that combines with the coil inductance to produce an LC resonance. Typically, such resonances unfortunately often lie within the band of the broadband RF signal. The effect of the shunt resistance is to reduce the Q of the LC resonance, thereby blunting the sharpness of any in-band resonances.
A reduction in the number of turns of the wire coil can push any LC resonances above the passband, but this reduction will also result in a reduction in inductance, limiting the suitability of the choke at the 5 MHz low end of the band. The presence of the shunt resistor in the above-described choke also reduces the signal impedance to ground, thereby increasing the signal loss.
In addition to the effects on frequency response, the RF chokes used in the equipment of the cable system must be capable of passing several amperes of AC current. The wire used for the coil must therefore be large enough to carry relatively high currents, usually up to 15 amperes in such cable transmission systems, without becoming excessively warm. Unfortunately, the larger the wire size the more troublesome the related parasitic resonance problem becomes. High currents also pose problems in that core materials are likely to approach saturation, thereby presenting the RF signals with an impedance which varies at the frequency rate of the single phase AC power signal. The effect of this is an unwanted modulation of RF signals commonly referred to as "hum mod".
The above described problems related to high AC current can be effectively reduced by careful selection of wire size, core material, core geometry, shunt resistors and winding dimensions. Many RF chokes have been used to give good performance to the 5 MHz to 450 MHz frequency range. However, when these chokes are used for the 5 MHz to 1,000 MHz frequency range, they exhibit a moderate amount of insertion losses at about 750 MHz. High attenuation of signal results when the losses are allowed to cascade over many circuits. It is desirable to maximize the reduction of insertion losses as many chokes are cascaded over large networks. Thus, savings of even the smallest amount of insertion loss manifests into a substantial amount of power savings over a large network.
Cable system capabilities are needed for extended bandwidths and upper frequency limits beyond 750 MHz to 1 GHz and higher. Therefore, a need exists for an improved RF choke which overcomes the problems and shortcomings associated with the prior art.